Wednesday, August 29, 2012

I bought a 1970 Evinrude 6hp outboard to power my Sunbird to and from the ramp, and for when there is no wind. I chose this motor because the price was right: A few hundred $$ vs. $1,000- $1,400 for a new one. Also, it was in very good condition. In addition, it weighs only 43 pounds, while newer ones I looked at were from 53 to 85 pounds. The weight advantage comes not from flimsy construction - in fact the Evinrude is impressively built - but from the fact that it is a simple 2 stroke design, unlike the modern 4 strokes. The Evinrude is not as clean burning as newer designs, but this is a motor that will see maybe 10 hours use a season. In addition, the motor needs to be removed for trailering. The far more expensive, and far heavier newer motors simply do not make sense for me, even if they are cleaner burning. The Evinrude has other advantages, too. As a 2 cylinder 2 stroke, it has 2 power pulses for each crankshaft revolution, vs. only 1 power pulse for 2 revolutions in a single cylinder 4 stroke. It produces it's rated power at lower RPM's than the 4 strokes, and is very smooth at all speeds.

It is a good idea to check the water pump on these motors, and I found my impeller to be worn out, and the pump housing corroded. Impellers are cheap enough, but housings are not. I decided to save some money and bore my housing out, using my Harbor Freight Mini Mill. I made a stainless steel liner from my scrapped Craftsman air compressor. By lucky coincidence, their inside diameters were the same. At right is a picture of the bored out housing and new liner.

Here is the liner installed. It is a press fit, and secured with J-B Weld. Will it last another 42 years? I'm not sure about that, but it will likely last long enough. I believe this housing, and most other parts are original on this low-time engine.

The inside of the lower end was in very good condition. I snapped the two rear bolts while removing them, for corrosion held them fast. Once again, I saw no sign that these were ever removed since new. I was able to extract them, and found that the holes are 1/4" deeper than the 1" bolts used. Since these threads were weakened by the corrosion, I replaced the bolts with new 1-1/4" stainless bolts, taking advantage of the extra threads at the bottom of the hole. Since I plan to inspect the lower end annually, I don't expect the new bolts to have time to seize in the holes.

One other problem I had with this engine was a very stiff throttle. I traced this to the bottom bearing of the vertical throttle shaft. Here they used a nylon bushing which had very close clearances to both the engine frame and the bevel gear. Some aluminum corrosion built up between the bushing and frame, nearly seizing it completely. It took a lot of effort to wiggle it free. Once clear of corrosion, I lubricated it with silicon grease, and it operates smoothly once again.

If you have this problem, be sure this hole is clear of corrosion, or it will come back. I rolled up sandpaper and drew it through a few times.Here is the vertical shaft. The stainless clip goes on the top end, and the nylon link connects it to the bevel gear. The bevel gear on the horizontal shaft is shown below right, and the shaft is below left. The shaft is solid brass, and is in much better condition that it appears. The bevel gear is aluminum, and also in great shape. The screw pinches it to the shaft, but even if it is loose, it will not turn because both the gear and shaft have a flat side

The next thing I looked at was the tilt mechanism. Initially very stiff, it soon loosened to the point where the motor would fall back down. I disassembled it to see how it worked. It uses a plastic cone for friction and a very thick spring to put compressive force on the cone.

Not shown in this picture is a flat fiberglass washer that is on the spring side of the tilt mechanism. It is a thrust washer that prevents the aluminum pieces from rubbing against each other. The spring and bolt were originally aluminized. I treated the rusted areas with rust converter, and then painted them. I put silicon grease on the plastic cone when I reassembled it.

Taking the flywheel off proved to be a challenge. Like nearly
everything else on this motor, it appeared to be undisturbed for the
last 42 years. I bolted on my Harbor Freight puller, and it did not
budge, even though the 1/4-20 stainless bolts I screwed into the flywheel were bending
under the load. I tried a few gentle taps, and nothing. I heated the
flywheel, still nothing. Finally, I supported the flywheel with 2 metal
bars supported by sawhorses. Now I could safely whack the top of the
puller without worrying about shock loads on the crank and bearings.
That worked. and I didn't have to hit it very hard.

Removing the flywheel exposed an ignition system in pristine condition. Everything looked original. I replaced the condensers, but being cheap, I filed the points. They are working well, and may last many hours.

The points are set at 0.020", and care should be taken to be precise, for the point gap affects the ignition timing. Even better, use a timing light. If the Set Points mark is between the two marks on the engine, it is correct. Time the top plug with the Set Points T and the bottom with the Set Points B. 1,000 RPM is recommended, but is not critical as the timing does not advance with RPM relative to the timing marks.

The timing does advance with throttle setting, but since the timing
marks move with the throttle, the timing light is accurate at any
throttle setting. Low speed operation RPM is primarily controlled by
the timing. The carburetor's throttle plate is at it's idle setting
throughout the low speed range, while the timing varies. At higher
speeds, the timing and throttle plate move together.

Some pictures of the powerhead. A few screw heads are rusty, but that's about it.

This motor is remarkably easy to start, and runs very well throughout it's speed range. It may be 42 years old, but it runs like new. It was a good choice for my application, and it's age doesn't worry me a bit.